This invention relates to a series of tetracyclic derivatives, to processes for their preparation, pharmaceutical compositions containing them, and their use as therapeutic agents. In particular, the invention relates to tetracyclic derivatives which are potent and selective inhibitors of cyclic guanosine 3xe2x80x2,5xe2x80x2-monophosphate specific phosphodiesterase (cGMP specific PDE) having utility in a variety of therapeutic areas where such inhibition is thought to be beneficial, including the treatment of cardiovascular disorders.
Thus, according to a first aspect, the present invention provides compounds of formula (I) 
and salts and solvates (e.g. hydrates) thereof, in which:
R0 represents hydrogen, halogen or C1-6 alkyl;
R1 represents hydrogen, C1-6alkyl, C2-6 alkenyl, C2-6 alkynyl, haloC1-6alkyl, C3-8cycloalkyl, C3-8cycloalkylC1-3alkyl, arylC1-3alkyl or heteroarylC1-3alkyl;
R2 represents an optionally substituted monocyclic aromatic ring selected from benzene, thiophene, furan and pyridine or an optionally substituted bicyclic ring 
xe2x80x83attached to the rest of the molecule via one of the benzene ring carbon atoms and wherein the fused ring A is a 5- or 6-membered ring which may be saturated or partially or fully unsaturated and comprises carbon atoms and optionally one or two heteroatoms selected from oxygen, sulphur and nitrogen; and
R3 represents hydrogen or C1-3 alkyl, or R1 and R3 together represent a 3- or 4-membered alkyl or alkenyl chain.
There is further provided by the present invention a subgroup of compounds of formula (I), the subgroup comprising compounds of formula (Ia) 
and salts and solvates (e.g. hydrates) thereof, in which:
R0 represents hydrogen, halogen or C1-6 alkyl;
R1 represents hydrogen, C1-6alkyl, haloC1-6alkyl, C3-8cycloalkyl, C3-8cycloalkylC1-3alkyl, arylC1-3alkyl or heteroarylC1-3alkyl; and
R2 represents an optionally substituted monocyclic aromatic ring selected from benzene, thiophene, furan and pyridine or an optionally substituted bicyclic ring 
xe2x80x83attached to the rest of the molecule via one of the benzene ring carbon atoms and wherein the fused ring A is a 5- or 6-membered ring which may be saturated or partially or fully unsaturated and comprises carbon atoms and optionally one or two heteroatoms selected from oxygen, sulphur and nitrogen.
Within R1 above, the term xe2x80x9carylxe2x80x9d as part of an arylC1-3alkyl group means phenyl or phenyl substituted by one or more (e.g. 1, 2 or 3) substituents selected from halogen, C1-6alkyl, C1-6alkoxy and methylenedioxy. The term xe2x80x9cheteroarylxe2x80x9d as part of a heteroarylC1-3alkyl group means thienyl, furyl or pyridyl each optionally substituted by one or more (e.g. 1, 2 or 3) substituents selected from halogen, C1-6 alkyl and C1-6alkoxy. The term xe2x80x9cC3-8cycloalkylxe2x80x9d as a group or part of a C3-8cycloalkylC1-3alkyl group means a monocyclic ring comprising three to eight carbon atoms. Examples of suitable cycloalkyl rings include the C3-6cycloalkyl rings cyclopropyl, cyclobutyl, cyclopentyl and cyclohexyl.
Within R2 above, optional benzene ring substituents are selected from one or more (e.g. 1, 2 or 3) atoms or groups comprising halogen, hydroxy, C1-6alkyl, C1-6alkoxy, xe2x80x94CO2Rb, haloC1-6alkyl, haloC1-6alkoxy, cyano, nitro and NRaRb, where Ra and Rb are each hydrogen or C1-6alkyl, or Ra may also represent C2-7alkanoyl or C1-6alkylsulphonyl. Optional substituents for the remaining ring systems are selected from one or more (e.g. 1, 2 or 3) atoms or groups comprising halogen, C1-6alkyl, C1-6alkoxy and arylC1-3alkyl as defined above. The bicyclic ring 
may, for example, represent naphthalene, a heterocycle such as benzoxazole, benzothiazole, benzisoxazole, benzimidazole, quinoline, indole, benzothiophene or benzofuran or 
(where n is an integer 1 or 2 and X and Y may each represent CH2, O, S or NH).
In the above definitions, the term, xe2x80x9calkylxe2x80x9d as a group or part of a group means a straight chain or, where available, a branched chain alkyl moiety. For example, it may represent a C1-4alkyl function as represented by methyl, ethyl, n-propyl, i-propyl, n-butyl, s-butyl and t-butyl. The term xe2x80x98alkenylxe2x80x99 as used herein includes straight-chained and branched alkenyl groups, such as vinyl and allyl groups. The term xe2x80x98alkynylxe2x80x99 as used herein includes straight-chained and branched alkynyl groups, suitably acetylene. The term xe2x80x9chalogenxe2x80x9d herein means a fluorine, chlorine, bromine or iodine atom. The term xe2x80x9chaloC1-6alkylxe2x80x9d means an alkyl group as defined above comprising one to six carbon atoms substituted at one or more carbon atoms by one or more (e.g. 1, 2 or 3) halogen atoms. Similarly, a haloC1-6alkoxy group is a haloC1-3alkyl group as defined above linked to the R2 benzene ring via an oxygen atom. Examples of haloC1-6alkyl groups include trifluoromethyl and 2,2,2-trifluoroethyl. An example of a haloC1-6alkoxy group is trifluoromethoxy. The term xe2x80x9cC2-7alkanoylxe2x80x9d means a C1-3alkylcarbonyl group where the C1-3alkyl portion is as defined above. An example of a suitable C2-7alkanoyl group is the C2alkanoyl group acetyl.
It will be appreciated that when R0 is a halogen atom or a C1-6alkyl group this substituent may be sited at any available position on the phenyl portion of the tetracyclic ring. However, a particular site of attachment is the ring 10-position.
The compounds of formula (I) may contain two or more asymmetric centres and thus can exist as enantiomers or diastereoisomers. In particular, in formula (I) above two ring chiral centres are denoted with asterisks. It is to be understood that the invention includes both mixtures and separate individual isomers of the compounds of formula (I).
The compounds of formula (I) may also exist in tautomeric forms and the invention includes both mixtures and separate individual tautomers thereof.
The pharmaceutically acceptable salts of the compounds of formula (I) which contain a basic centre are acid addition salts formed with pharmaceutically acceptable acids. Examples include the hydrochloride, hydrobromide, sulphate or bisulphate, phosphate or hydrogen phosphate, acetate, benzoate, succinate, fumarate, maleate, lactate, citrate, tartrate, gluconate, methanesulphonate, benzenesulphonate and p-toluenesulphonate salts. Compounds of the formula (I) can also provide pharmaceutically acceptable metal salts, in particular alkali metal salts, with bases. Examples include the sodium and potassium salts.
A particular group of compounds of the invention are those compounds of formula (I) in which R0 is hydrogen or halogen (e.g. fluorine), especially hydrogen.
Another particular group of compounds of the invention are those compounds of formula (I) in which R1 represents hydrogen, C1-4alkyl, haloC1-4alkyl, C3-6cycloalkyl, C3-6cycloalkylmethyl, pyridylC1-3alkyl, furylC1-3alkyl or optionally substituted benzyl. Within this particular group of compounds, examples of C1-4alkyl groups are methyl, ethyl, n-propyl, i-propyl and n-butyl. Examples of C3-6cycloalkylmethyl groups are cyclopropylmethyl and cyclohexylmethyl. Examples of optionally substituted, benzyl groups include benzyl and halobenzyl (e.g. fluorobenzyl).
A further particular group of compounds of the invention are those compounds of formula (I) in which R2 represents an optionally substituted benzene, thiophene, furan, pyridine or naphthalene ring or an optionally substituted bicyclic ring 
(where n is 1 or 2 and X and Y are each CH2 or O). Within this particular group of compounds, examples of substituted benzene groups are benzene substituted by one of halogen (e.g. chlorine), hydroxy, C1-3alkyl (e.g. methyl, ethyl or i-propyl), C1-3alkoxy (e.g. methoxy or ethoxy), xe2x80x94CO2Rb, halomethyl (e.g. trifluoromethyl), halomethoxy (e.g. trifluoromethoxy), cyano, nitro or NRaRb where Ra and Rb are each hydrogen or methyl or Ra is acetyl; or benzene substituted by dihalo (e.g. dichloro) or by C1-3alkoxy (e.g. methoxy) and one of halogen (e.g. chlorine) and hydroxy. An example of a substituted thiophene ring is a halo (e.g. bromo) substituent thiophene ring.
A still further particular group of compounds of formula I are those wherein R3 represents hydrogen or R1 and R3 together represent a 3-membered alkyl chain.
A preferred group of compounds of the invention are the cis isomers of formula (I) represented by formula (Ib) 
and mixtures thereof with their cis optical enantiomers, including racemic mixtures, and salts and solvates (e.g. hydrates) of these compounds in which R0 is hydrogen or halogen (e.g. fluorine), especially hydrogen and R1, R2 and R3 are as defined previously.
The single isomers represented by formula (Ib), i.e. the 6R,12aR isomers, are particularly preferred.
Within the above definitions R1 may preferably represent C1-4alkyl (e.g. methyl, ethyl, i-propyl and n-butyl), C3-6cycloalkyl (e.g. cyclopentyl) or C3-6cycloalkylmethyl (e.g. cyclopropylmethyl).
R2 may preferably represent a substituted benzene ring such as benzene substituted by C1-3alkoxy (e.g. methoxy) or by C1-3alkoxy (e.g. methoxy) and halogen (e.g. chlorine), particularly 4-methoxyphenyl or 3-cloro-4-methoxyphenyl, or R2 may preferably represent 3,4-methylenedioxyphenyl.
It is to be understood that the present invention covers all appropriate combinations of particular and preferred groupings hereinabove.
Particular individual compounds of the invention include:
Cis-2,3,6,7,12,12a-Hexahydro-2-(4-pyridylmethyl)-6-(3,4-methylenedioxyphenyl)pyrazino[2xe2x80x2,1xe2x80x2:6,1]pyrido[3,4-b]indole-1,4-dione;
Cis-2,3,6,7,12,12a-Hexahydro-6-(2,3-dihydrobenzo[b]furan-5-yl)-2-methyl-pyrazino[2xe2x80x2,1xe2x80x2:6,1]pyrido[3,4b]indole-1,4-dione;
Cis-2,3,6,7,12,12a-Hexahydro-6-(5-bromo-2-thienyl)-2-methyl-pyrazino[2xe2x80x2,1xe2x80x2:6,1]pyrido[3,4-b]indole-1,4-dione;.
Cis-2,3,6,7,12,12a-Hexahydro-2-butyl-6-(4-methylphenyl)-pyrazino[2xe2x80x2,1xe2x80x2:6,1pyrido[3,4-b]indole-1,4-dione;
(6R,12aR)-2,3,6,7,12,12a-Hexahydro-2-isopropyl-6-(3,4-methylenedioxyphenyl)-pyrazino[2xe2x80x2,1xe2x80x2:6,1]pyrido[3,4b]indole-1,4-dione;
(6R,12aR)-2,3,6,7,12,12a-Hexahydro-2-cyclopentyl-6-(3,4-methylenedioxyphenyl)pyrazino[2xe2x80x2,1xe2x80x2:6,1]pyrido[3,4-b]indole-1,4-dione;
(6R,12aR)-2,3,6,7,12,12a-Hexahydro-2-cyclopylmethyl-6-(4-methoxyphenyl)-pyrazino(2xe2x80x2,1xe2x80x2:6,1]pyrido[3,4b]indole-1,4dione;
(6R,12aR)-2,3,6,7,12,12a-Hexahydro-6-(3-chloro-4-methoxyphenyl)-2-methyl-pyrazino[2xe2x80x2,1xe2x80x2:6,1]pyrido[3,4-b]indole-1,4-dione;
(6R,12aR)-2,3,6,7,12,12a-Hexahydro-2-methyl-6-(3,4-methylenedioxyphenyl)-pyrazino[2xe2x80x2,1xe2x80x2:6,1]pyrido[3,4-b]indole-1,4-dione;
(6R,12aR)-2,3,6,7,12,12a-Hexahydro-6-(3,4-methylenedioxyphenyl)-pyrazino[2xe2x80x2,1xe2x80x2:6,1]pyrido[3,4-b]indole-1,4-dione;
(5aR,12R,14aS)-1,2,3,5,6,11,12,14a-Octahydro-12-(3,4-methylenedioxyphenyl)-pyrrolo[1xe2x80x3,2xe2x80x3:4xe2x80x2,5xe2x80x2]pyrazino[2xe2x80x2,1xe2x80x2:6,1lpyrido[3,4-b]indole-5-1,4-dione;
and physiologically acceptable salts and solvates (e.g. hydrates) thereof.
A specific compound of the invention is:
(6R,12aR)-2,3,6,7,12,12a-Hexahydro-2-methyl-6-(3,4-methylenedioxyphenyl)-pyrazino[2xe2x80x2,1:6,1]pyrido[3,4-b]indole-1,4-dione;
and physiologically acceptable salts and solvates (e.g. hydrates) thereof.
It has been shown that compounds of the present invention are potent and selective inhibitors of cGMP specific PDE. Thus, compounds of formula (I) are of interest for use in therapy, specifically for the treatment of a variety of conditions where inhibition of cGMP specific PDE is thought to be beneficial.
As a consequence of the selective PDE V inhibition exhibited by compounds of the present invention, cGMP levels are elevated, which in turn can give rise to beneficial anti-platelet, anti-neutrophil, anti-vasospastic, vasodilatory, natriuretic and diuretic activities as well as potentiation of the effects of endothelium-derived relaxing factor (EDRF), nitrovasodilators, atrial natriuretic factor (ANF), brain natriuretic peptide (BNP), C-type natriuretic peptide (CNP) and endothelium-dependent relaxing agents such as bradykinin, acetylcholine and 5-HT1. The compounds of formula (I) therefore have utility in the treatment of a number of disorders, including stable, unstable and variant (Prinzmetal) angina, hypertension, pulmonary hypertension, congestive heart failure, renal failure, atherosclerosis, conditions of reduced blood vessel patency (e.g. post-percutaneous transluminal coronary angioplasty), peripheral vascular disease, vascular disorders such as Raynaud""s disease, inflammatory diseases, stroke, bronchitis, chronic asthma, allergic asthma, allergic rhinitis, glaucoma and diseases characterised by disorders of gut motility (e.g. irritable bowel syndrome).
It will be appreciated that references herein to treatment extend to prophylaxis as well as treatment of established conditions.
It will also be appreciated that xe2x80x98a compound of formula (I),xe2x80x99 or a physiologically acceptable salt or solvate thereof can be administered as the raw compound, or as a pharmaceutical composition containing either entity.
There is thus provided as a further aspect of the invention a compound of formula (I) for use in the treatment of stable, unstable and variant (Prinzmetal) angina, hypertension, pulmonary hypertension, chronic obstructive pulmonary disease, congestive heart failure, renal failure, atherosclerosis, conditions of reduced blood vessel patency, (e.g. post-PTCA), peripheral vascular disease, vascular disorders such as Raynaud""s disease, inflammatory diseases, stroke, bronchitis, chronic asthma, allergic asthma, allergic rhinitis, glaucoma or diseases characterised by disorders of gut motility (e.g. IBS).
According to another aspect of the invention, there is provided the use of a compound of formula (I) for the manufacture of a medicament for the treatment of stable, unstable and variant (Prinzmetal) angina, hypertension, pulmonary hypertension, chronic obstructive pulmonary disease, congestive heart failure, renal failure, atherosclerosis, conditions of reduced blood vessel patency, (e.g. post-PTCA), peripheral vascular disease, vascular disorders such as Raynaud""s disease, inflammatory diseases, stroke, bronchitis, chronic asthma, allergic asthma, allergic rhinitis, glaucoma or diseases characterised by disorders of gut motility (e.g. IBS).
In a further aspect, the invention provides a method of treating stable, unstable and variant (Prinzmetal) angina, hypertension, pulmonary hypertension, chronic obstructive pulmonary disease, congestive heart failure, renal failure, atherosclerosis, conditions of reduced blood vessel patency, (e.g. post-PTCA), peripheral vascular disease, vascular disorders such as Raynaud""s disease, inflammatory diseases, stroke, bronchitis, chronic asthma, allergic asthma, allergic rhinitis, glaucoma or diseases characterised by disorders of gut motility (e.g. IBS) in a human or non-human animal body which comprises administering to said body a therapeutically effective amount of a compound with formula (I).
Compounds of the invention may be administered by any suitable route, for example by oral, buccal, sub-lingual, rectal, vaginal, nasal, topical or parenteral (including intravenous, intramuscular, subcutaneous and intracoronary) administration. Oral administration is generally preferred.
For administration to man in the curative or prophylactic treatment of the disorders identified above, oral dosages of a compound of formula (I) will generally be in the range of from 0.5-800 mg daily for an average adult patient (70 kg). Thus for a typical adult patient, individual tablets or capsules contain from 0.2-400 mg of active compound, in a suitable pharmaceutically acceptable vehicle or carrier, for administration in single or multiple doses, once or several times per day. Dosages for intravenous, buccal or sublingual administration will typically be within the range of from 0.1-400 mg per single dose as required. In practice the physician will determine the actual dosing regimen which will be most suitable for an individual patient and it will vary with the age, weight and response of the particular patient. The above dosages are exemplary of the average case but there can be individual instances in which higher or lower dosage ranges may be merited, and such are within the scope of this invention.
For human use, a compound of the formula (I) can be administered alone, but will generally be administered in admixture with a pharmaceutical carrier selected with regard to the intended route of administration and standard pharmaceutical practice. For example, the compound may be administered orally, buccally or sublingually, in the form of tablets containing excipients such as starch or lactose, or in capsules or ovules either alone or in admixture with excipients, or in the form of elixirs or suspensions containing flavouring or colouring agents. Such liquid preparations may be prepared with pharmaceutically acceptable additives such as suspending agents (e.g. methylcellulose, a semi-synthetic glyceride such as witepsol or mixtures of glycerides such as a mixture of apricot kernel oil and PEG-6 esters or mixtures of PEG-8 and caprylic/capric glycerides). A compound may also be injected parenterally, for example intravenously, intramuscularly, subcutaneously or intracoronarily. For parenteral administration, the compound is best used in the form of a sterile aqueous solution which may contain other substances, for example salts, or monosaccharides such as mannitol or glucose, to make the solution isotonic with blood.
Thus, the invention provides in a further aspect a pharmaceutical composition comprising a compound of the formula (I) together with a pharmaceutically acceptable diluent or carrier therefor.
There is further provided by the present invention a process of preparing a pharmaceutical composition comprising a compound of formula (I), which process comprises mixing a compound of formula (I) together with a pharmaceutically acceptable diluent or carrier therefor.
A compound of formula (I) may also be used in combination with other therapeutic agents which may be useful in the treatment of the above-mentioned disease states. The invention thus provides, in another aspect, a combination of a compound of formula (I) together with another therapeutically active agent.
The combination referred to above may conveniently be presented for use in the form of a pharmaceutical formulation and thus pharmaceutical compositions comprising a combination as defined above together with a pharmaceutically acceptable diluent or carrier comprise a further aspect of the invention.
The individual components of such a combination may also be administered either sequentially or simultaneously in separate pharmaceutical formulations.
Appropriate doses of known therapeutic agents for use in combination with a compound of formula (I) will be readily appreciated by those skilled in the art.
Compounds of formula (I) may be prepared by any suitable method known in the art or by the following processes which form part of the present invention. In the methods below R0, R1 and R2 are as defined in formula (I) above unless otherwise indicated.
Thus, a process (A) for preparing a compound of formula (I) wherein R3 represents hydrogen comprises treating a compound of formula (II) 
(in which Alk represents C1-6alkyl, e.g. methyl or ethyl and Hal is a halogen atom, e.g. chlorine) with a primary amine R1NH2 in a suitable solvent such as an alcohol (e.g. methanol or ethanol) or a mixture of solvents, conveniently at a temperature of from 20xc2x0 C. to reflux (e.g. at about 50xc2x0 C.).
A compound of formula (II) may conveniently be prepared by treating a compound of formula (III) 
with a haloacetyl halide (e.g. chloroacetyl chloride) in a suitable solvent such as a halogenated hydrocarbon (e.g. trichloromethane or dichloromethane), or an ether (e.g. tetrahydrofuran), preferably in the presence of a base such as an organic amine (e.g. a trialkylamine such as triethylamine) or an alkali metal carbonate or bicarbonate (e.g. NaHCO3). The reaction may conveniently be effected at a temperature of from xe2x88x9220xc2x0 C. to +20xc2x0 C. (e.g. at about 0xc2x0 C.).
A compound of formula (I) may also be prepared from a compound of formula (III) in a two-step procedure via a compound of formula (II) isolated without purification.
Compounds of formula (I) may be prepared as individual enantiomers in two steps from the appropriate enantiomer of formula (III) or as mixtures (e.g. racemates) of either pairs of cis or trans isomers from the corresponding mixtures of either pairs of cis or trans isomers of formula (III).
Individual enantiomers of the compounds of the invention may be prepared from racemates by resolution using methods known in the art for the separation of racemic mixtures into their constituent enantiomers, for example using HPLC (high performance liquid chromatography) on a chiral column such as Hypersil naphthylurea.
A compound of formula (III) may conveniently be prepared from a tryptophan alkyl ester of formula (IV) 
(where Alk is as previously defined) or a salt thereof (e.g. the hydrochloride salt) according to either of the following procedures (a) and (b). Procedure (b) is only suitable for preparing cis isomers of formula (III) and may be particularly suitable for preparing individual cis enantiomers of formula (III) from D- or L-tryptophan alkyl esters as appropriate.
This comprises a Pictet-Spengler cyclisation between a compound of formula (IV) and an aldehyde R2CHO. The reaction may conveniently be effected in a suitable solvent such as a halogenated hydrocarbon (e.g. dichloromethane) or an aromatic hydrocarbon (e.g. toluene) in the presence of an acid such as trifluoroacetic acid. The reaction may conveniently be carried out at a temperature of from xe2x88x9220xc2x0 C. to reflux to provide a compound of formula (III) in one step. The reaction may also be carried out in a solvent such as an aromatic hydrocarbon (e.g. benzene or toluene) under reflux, optionally using a Dean-Stark apparatus to trap the water produced.
The reaction provides a mixture of cis and trans isomers which may be either individual enantiomers or racemates of pairs of cis or trans isomers depending upon whether racemic or enantiomerically pure tryptophan alkyl ester was used as the starting material. Individual cis or trans enantiomers may conveniently be separated from mixtures thereof by fractional crystallisation or by chromatography (e.g. flash column chromatography) using appropriate solvents and eluents. Similarly, pairs of cis and trans isomers may be separated by chromatography (e.g. flash column chromatography) using appropriate eluents. An optically pure trans isomer may also be converted to an optically pure cis isomer using suitable epimerisation procedures. One such procedure comprises treating the trans isomer or a mixture (e.g. 1:1 mixture) of cis and trans isomers with methanolic or aqueous hydrogen chloride at a temperature of from 0xc2x0 C. to the refluxing temperature of the solution. The mixture may then be subjected to chromatography (e.g. flash column chromatography) to separate the resulting diastereoisomers, or in the procedure utilising aqueous hydrogen chloride the desired cis isomer precipitates out as the hydrochloride salt which may then be isolated by filtration.
This comprises a four-step procedure from a compound of formula (IV) or a salt thereof (e.g. the hydrochloride salt). The procedure is particularly suitable for preparing a 1R. 3R isomer of formula (III) from a D-tryptophan alkyl ester of formula (IV) or a salt thereof (e.g. the hydrochloride salt). Thus, a first step (i) comprises treating a compound of formula (IV) with an acid halide R2COHal (where Hal is as previously defined) in the presence of a base, e.g. an organic base such as a trialkylamine (for example triethylamine), to provide a compound of formula (V) 
The reaction may be conveniently carried out in a suitable solvent such as a halogenated hydrocarbon (e.g. dichloromethane) or an ether (e.g. tetrahydrofuran) and at a temperature of from xe2x88x9220xc2x0 C. to +40xc2x0 C.
Step (ii) comprises treating a compound of formula (V) with an agent to convert the amide group to a thioamide group. Suitable sulfurating agents are well-known in the art. Thus, for example, the reaction may conveniently be effected by treating (V) with Lawesson""s reagent. This reaction may conveniently be carried out in a suitable solvent such as an ether (e.g. dimethoxyethane) or an aromatic hydrocarbon (e.g. toluene) at an elevated temperature such as from 40xc2x0 C. to 80xc2x0 C. to provide a compound of formula (VI) 
Step (iii) comprises treating a compound of formula (VI) with a suitable agent to provide a compound of formula (VII) 
(where Hal is a halogen atom, e.g. iodine). The reaction may conveniently be effected by treating (VI) with an alkylating agent such as a methyl halide (e.g. methyl iodide) or an acylating agent such as an acetyl halide (e.g. acetyl chloride) in a suitable solvent such as a halogenated hydrocarbon (e.g. dichloromethane) at an elevated temperature (e.g. under reflux).
In step (iv) the resulting iminium halide of formula (VII) may be treated with a reducing agent such as boron hydride, e.g. sodium borohydride, to provide the desired compound of formula (III). The reduction may conveniently be effected at a low temperature, e.g. within the range of xe2x88x92100xc2x0 C. to 0xc2x0 C., in a suitable solvent such as an alcohol (e.g. methanol).
There is further provided by the present invention a process (B) for preparing a compound of formula (I), wherein R1 and R3 together represent a 3- or 4-membered alkyl or alkenyl chain, which process (B) comprises cyclisation of a compound of formula (VIII) 
wherein Alk represents C1-6alkyl and R1 and R3 together represent a 3- or 4-membered chain both as hereinbefore described. The cyclisation is suitably carried out in an organic solvent or solvents, such as an alcoholic solvent (e.g. methanol) and optionally an ether solvent such as tetrahydrofuran, and in the presence of a reducing agent, aptly a palladium catalyst, such as palladium on carbon.
Conveniently a compound of formula (VIII) is prepared by reaction of a compound of formula (III) as hereinbefore described with a compound of formula (IX) 
wherein Hal represents a halogen atom as hereinbefore described, R1 and R3 together represent a 3- or 4-membered chain as hereinbefore described and R4 represents a protecting group, suitably a benzyloxycarbonyl group or the like. Typically the reaction is carried out in a chlorinated organic solvent, such as dichloromethane, and a tertiary amine, such as triethylamine or the like.
According to a further aspect of the present invention, there is provided a process (C) for preparing a compound of formula (I) wherein R3 represents C1-3alkyl, which process comprises cyclisation of a compound of formula (X) 
wherein Alk represents C1-6alkyl as hereinbefore described and R5 represents C2-5alkyl, substituted at C1 by a halogen atom, the halogen atom being as hereinbefore described. Suitably the cyclisation is achieved by reflux for many hours, such as 22 to 26 hours, in the presence of an ether solvent, such as tetrahydrofuran, and a suitable amine as hereinafter described in the accompanying examples.
Aptly a compound of formula (X) can be prepared from a compound of formula (III) by suitable acylation techniques, such as reaction with a C3-6carboxylic acid, substituted at C2 by a halogen atom in a halogenated organic solvent, such as dichloromethane.
Compounds of formula (I) may be converted to other compounds of formula (I). Thus, for example, when R2 is a substituted benzene ring it may be necessary or desirable to prepare the suitably substituted compound of formula (I) subsequent to process (A), (B) or (C) as above. Examples of appropriate interconversions include nitro to amino or aralkyloxy to hydroxy by suitable reducing means (e.g. using a reducing agent such as SnCl2 or a palladium catalyst, such as palladium-on-carbon), or amino to substituted amino such as acylamino or sulphonylamino using standard acylating or sulphonylating conditions. In the case where R2 represents a substituted bicyclic system, suitable interconversion can involve removal of a substituent, such as by treatment with a palladium catalyst (e.g. palladium-on-carbon) whereby, for example, a benzyl substituent may be removed from a suitable bicyclic system.
The pharmaceutically acceptable acid addition salts of the compounds of formula (I) which contain a basic centre may be prepared in a conventional manner. For example, a solution of the free base may be treated with a suitable acid, either neat or in a suitable solution, and the resulting salt isolated either by filtration or by evaporation-under vacuum of the reaction solvent. Pharmaceutically acceptable base addition salts may be obtained in an analogous manner by treating a solution of a compound of formula (I) with a suitable base. Both types of salt may be formed or interconverted using ion-exchange resin techniques.
Compounds of the invention may be isolated in association with solvent molecules by crystallisation from or evaporation of an appropriate solvent
Thus, according to a further aspect of the invention, we provide a process for preparing a compound of formula (I) or a salt or solvate (e.g. hydrate) thereof which comprises process (A), (B) or (C) as hereinbefore described followed by
i) an interconversion step; and/or either
ii) salt formation; or
iii) solvate (e.g. hydrate) formation.
There is further provided by the present invention compounds of formulae (II), (VII), (X) and further compounds of formulae (III), (V, (VI) and (VII), with the exception for compounds (III), (V), (VI) and (VII) wherein R0 is hydrogen, R2 is phenyl and Alk is methyl.